[Devel] [PATCH rh7] ms/pidns: fix NULL dereference in __task_pid_nr_ns()

[Andrey Ryabinin]

From: Eric Dumazet <edumazet at google.com>

commit 81b1a832d79749058863cffe2c0ed4ef40f6e6ec upstream.

I got a crash during a "perf top" session that was caused by a race in
__task_pid_nr_ns() :

pid_nr_ns() was inlined, but apparently compiler chose to read
task->pids[type].pid twice, and the pid->level dereference crashed
because we got a NULL pointer at the second read :

    if (pid && ns->level <= pid->level) { // CRASH

Just use RCU API properly to solve this race, and not worry about "perf
top" crashing hosts :(

get_task_pid() can benefit from same fix.

Signed-off-by: Eric Dumazet <edumazet at google.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

https://jira.sw.ru/browse/PSBM-75247
Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 kernel/pid.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index 4f8d1d6d50fa..d1f9d4ccf9a5 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -511,7 +511,7 @@ struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 	rcu_read_lock();
 	if (type != PIDTYPE_PID)
 		task = task->group_leader;
-	pid = get_pid(task->pids[type].pid);
+	pid = get_pid(rcu_dereference(task->pids[type].pid));
 	rcu_read_unlock();
 	return pid;
 }
@@ -572,7 +572,7 @@ pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
 	if (likely(pid_alive(task))) {
 		if (type != PIDTYPE_PID)
 			task = task->group_leader;
-		nr = pid_nr_ns(task->pids[type].pid, ns);
+		nr = pid_nr_ns(rcu_dereference(task->pids[type].pid), ns);
 	}
 	rcu_read_unlock();
 
-- 
2.13.6